I think all the time we have Mixed abiity gruops and I don´t think its a problem. I think it could be helpful to the all group because when some students are able to do something in a different way or better than the others they try to copy the way the best students do.
I think there is a huge advantage having students with higer level or hability, because they don´t mock of the students with lower ability in fact they try to help the classmates to improve their abilities